but·ter·fish 
(bŭtər-fĭsh′)
(bŭtər-fĭsh′)n. pl. butterfish or but·ter·fish·es
1. Any of various fishes of the family Stromateidae having tender oily flesh, especially Peprilus triacanthus of the North American Atlantic coast.
2. Any of various other fishes having oily flesh or slippery skin.
[Originally applied to a species of gunnel found in British waters, noted for its slippery coating of mucus.]